In underground areas and the like, it is difficult in some cases to secure a good radio wave environment between a wireless base station and a portable terminal. For such cases, there is a known technology that enlarges the coverage area of a mobile communication system by installing a relay apparatus (repeater apparatus) that receives radio waves radiated by a wireless base station and that amplifies signals received and radiates the signals via an antenna connected to a repeater apparatus.
For example, PTL 1 describes a repeater apparatus that amplifies an input high-frequency signal and then sends out the amplified signal as a signal of the same frequency. PTL 2 describes a signal enhancer used to expand the coverage area of a wireless communication system.
When a plurality of repeater apparatuses are installed between a wireless base station and a terminal, there is possibility that the terminal may receive not only radio waves radiated from the repeater apparatuses but also reflected waves from structural bodies and the like in surrounding areas. In other words, when a plurality of coverage areas are formed by repeater apparatuses, influence of radio waves in adjacent coverage areas can in some cases make a complicated radio wave environment. In such a case, a terminal that communicates with the base station via repeater apparatuses suffers interference, so that unstable reception levels of radio waves radiated from the repeater apparatuses may result or path switching may frequently occur between the terminal and a plurality of repeater apparatuses. Consequently, there is possibility that the communication quality of the terminal may decline.
A typical repeater apparatus used to enlarge the coverage area of a mobile communication system directly amplifies and transfers a signal received from a base station or another repeater apparatus. Specifically, when a plurality of repeater apparatuses are installed for one base station, coverage areas newly formed by repeater apparatuses each have radio waves of the same frequencies radiated from the repeater apparatuses. Therefore, when repeater apparatuses are installed in an environment where radio wave interference easily occurs, there arises a problem that communication quality may easily reduce due to interference between radio waves of equal frequencies in adjacent coverage areas newly formed by the repeater apparatuses.
Furthermore, neither PTL 1 nor PTL 2 describes that arrangement of frequencies of cells in coverage areas newly formed by repeater apparatuses is set so that radio wave environments of the coverage areas become more preferable.